With recent advancement of digital technologies, electronic hardware such as portable information devices and home information appliances has been developed to provide higher functionality. As the electronic hardware has been developed to provide higher functionality, development of further miniaturized and higher-speed semiconductor elements which are built into the electronic hardware are progressing at a high pace. Among them, the use of large-capacity nonvolatile memories which are represented by a flash memory has been spreading at a rapid pace. Furthermore, as next-generation new nonvolatile memories which have a potential to replace the flash memory, a resistance variable nonvolatile memory device including so-called a resistance variable element (ReRAM) has been researched and developed. As defined herein, the resistance variable element refers to an element which has a characteristic in which a resistance value changes reversibly in response to electric signals and is able to store information corresponding to the resistance value in a nonvolatile manner. Unlike a phase change random access memory (PCRAM) which is adapted to change a resistance value due to a fact that a change of a crystalline state is induced by heat generated by electric stresses applied thereto, the resistance variable element changes its resistance value by changing redox states of a resistance variable material, directly in response to the electric stresses applied thereto, i.e., by migration of electrons.
As an example of a large-capacity nonvolatile memory incorporating the resistance variable elements, a cross-point nonvolatile memory element has been proposed. The cross-point nonvolatile memory element has a structure suitable for miniaturization, and an element including a resistance variable layer as a memory section and a non-linear element such as a varistor as a current controlling element is disclosed (e.g., see patent document 1).
FIG. 19 is a view showing a nonvolatile memory device including a conventional resistance variable element. FIG. 19 is a cross-sectional view of a memory cell 380 taken along the direction of a bit line 310, in a cross-point memory cell array including bit lines 310, word lines 320 and memory cells 380 formed at cross-points of the bit lines 310 and the word lines 320. A resistance variable element 360 includes a resistance variable layer 330 for storing data according to a change in an electric resistance because of electric stresses applied thereto, an upper electrode 340 and a lower electrode 350 sandwiching the resistance variable layer 330 between them. On the upper portion of the resistance variable element 360, there is provided a two-terminal non-linear element 370 having a nonlinear current-voltage characteristic for flowing a current bidirectionally. The memory cell 380 is constituted by a series circuit including the resistance variable element 360 and the non-linear element 370. The non-linear element 370 is a two-terminal element such as a diode, having a nonlinear current-voltage characteristic in which a current changes inconstantly with respect to a voltage change. The bit line 310 serving as an upper wire is electrically connected to the non-linear element 370. The word line 320 serving as a lower wire is electrically connected to the lower electrode 350 of the resistance variable element 360. A current flows bidirectionally through the non-linear element 370 when rewriting for the memory cell 380. For example, as the non-linear element 370, a varistor (ZnO or SrTiO3) having a current-voltage characteristic which is bidirectionally (both at positive voltage side and negative voltage side) symmetric and non-linear, is used. With the above configuration, it is possible to flow a current with a current density of 30 kA/cm2 or higher which is required for rewriting for the resistance variable element 360, and achieve a larger capacity.